It has been proposed to incorporate charge storage zones in pixels of an image sensor. For example, such charge storage zones permit the storage of charge carriers from a photodiode following a global shutter operation. As a further example, charge storage zones permit the storage of charge carriers in time-of-flight image sensors that are able to measure depth information from an image scene.
As the size of CMOS pixels is reduced, the space available for the charge storage zones is also being reduced. Charge storage zones of reduced size permit fewer electrons or holes to be stored, resulting in a degradation of the dynamic range of the pixel, and thus the signal to noise ratio.
It has been proposed to provide charge storage zones in the form of doped trenches that are laterally pinched by CDTIs (capacitive deep trench isolations) in order to hold charge carriers therein. There is a technical difficulty in extending the size of such trenches while maintaining a compact pixel layout, avoiding increasing the time lag for evacuating the charge, and/or losing the ability to read the entire storage zone.